Vaporization of unreacted cyclohexane from cyclohexane oxidation products

ABSTRACT

The present invention relates to a process for the oxidation of cyclohexane with molecular oxygen in the presence of boric acidtype additives to produce borate esters and is particularly concerned with vaporization of at least a part of the unreacted cyclohexane is the oxidation effluent under the conditions so as to minimize oxidation or oxygen transfer during this vaporization.

United States Patent 1111 3,607,905

[72] Inventor Joseph L. Russell [56] References Cited Rtildgewood, NJ.UNITED STATES PATENTS [21] 7 3,239,552 3/1966 Federeta! 260/462A F'Ied1968 3 243 449 3/l966 w' k 260/462A 45 Patented Sept.2l 1971 [73] Assinee Hflconlmem M I 3,346,614 10 1967 Starks et 31.. 260/462A 2 13,439,041 4/1969 .Geyetal 260/631 B Primary ExaminerLeon ZitverAssistant Examiner-Joseph E. Evans Attorneys-William C. Long, Mario A.Monaco, Morris [54] VAPORIZATION 0F UNREACTED W1seman and Dav1d DickCYCLOHEXANE FROM CYCLOHEXANE OXIDATION PRODUCTS 3 Clams 1 DrawingABSTRACT: The present invention relates to a process for [52] U.S.Cl260/462, the oxidation of cyclohexane with molecular oxygen in the260/586, 260/610, 260/631 presence of boric acid-type additives toproduce borate esters [Sl] Int. Cl C07f 5/04, and is particularlyconcerned with vaporization of at least a C07c 35/08 part of theunreacted cyclohexane is the oxidation effluent [50] Field of Search260/462 A, under the conditions so as to minimize oxidation or oxygen631 B transfer during this vaporization.

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True, MINUTES PATENTED $921 M1 3 01,905

MOL FRACTION OF PE'ROX/DE DECOMPOSED TIME) MINUTES ATT EV VAPORIZATIONOF UNREACTED CYCLOHEXANE FROM CYCLOHEXANE OXIDATION PRODUCTS BACKGROUNDOF THE INVENTION The oxidation of cyclohexane with molecular oxygen inthe presence of boric acid-type additives has assumed very greatimportance in the chemical field. The borate ester mixtures resultingfrom this oxidation are readily converted to compounds includingcyclohexanol and cyclohexanone which compounds in turn find greatutility, for example, in the production ofadipic acid caprolactum or thelike.

The oxidation of cyclohexane in the presence of the boric acid additivesis known and can be illustrated by reference to issued U.S. Pat. No.3,243,449. In oxidations of cyclohexane it j is generally necessary toprovide heat to the reaction zone in order to maintain proper conditionsfor the oxidation. One such method of providing heat involves avaporization of a portion of the cyclohexane which is being fed to theoxidation as fresh cyclohexane or as recycle condensate from theoxidation zones. U.S. Pat. No. 3,317,614 describes this operation.

In the oxidation of cyclohexane using a boric acid additive, it has beendetermined that it is particularly advantageous to subject the oxidationeffluent to a vaporization whereby vapors of unreacted cyclohexane areseparated and returned to the oxidation system. The reactor effluentmixtures comprise mainly unreacted cyclohexane together with an amountof unreacted boric acid additive and also contains borate ester,predominantly cyclohexyl meta borate, together with other cyclohexaneoxidation products including cyclohexanone and various acidic andneutral materials. Also contained in the effluent mixture is asubstantial quantity of peroxy oxygen containing materials, usuallycyclohexyl hydroperoxide or cyclohexyl perborate. It has been found thatthe heating of this effluent in order to separate at least part of thecyclohexane as vapor when carried out in accordance with usual andconventional procedures results in certain inefficiencies in overalloperation.

SUMMARY OF THE INVENTION In accordance with the present invention, ithas been discovered that the vaporization of the oxidation effluentmixtures above referrred to must be carried out under conditions so asto minimize oxygen transfer which occurs for example, through reductionof the peroxy oxygen compounds during the vaporization. Particularly, itis essential in accordance wit the present invention that thevaporization be carried out such that less than 50 percent of the peroxyoxygen is lost during the vaporization and preferably less than 30percent.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a plot ofpercentage peroxide reacting as a function of time. The various curvesillustrate the reaction at different temperatures.

DETAILED DESCRIPTION OF THE INVENTION Illustrative conditions oftemperature, pressure and amounts and types of boric acid additives usedin the oxidation are fully set forth in U.S. Pat. No. 3,243,449 which isabove referred to.

Practice of the present invention is applicable both to batch andcontinuous operation and in the latter case to continuous operationsinvolving use of a series of separate reaction zones or to the use ofatower reactor.

In accordance with the invention, the peroxy compound containingoxidation effluent usually containing 5 to vweight percent peroxycompound based on the total of cyclohexane oxidation products issubjected to vaporization wherein the effluent is heated either bydirect or indirect heat exchange. In the former case preheated vapor isdirectly contacted with the effluent (e.g. preheated nitrogen) in orderto effect the desired vaporization of the cyclohexane. In the lattercase, an indirect heat exchange is achieved through the use ofappropriate indirect heat exchange means such as heating coils, heatingjackets and the like known means.

As an essential aspect in preventing the excessive decomposition of theperoxy compounds in the vaporization zone, the residence time of theeffluent in the vaporization zone is maintained less than about 5minutes, at temperatures below 190 C. A preferred residence time in thevaporizationzone is 3 minutes or less, preferably 0.5 to 2.0 minutes.

In order to effect vaporization the effluent is heated to a temperatureof about 170 C. to 185 C. Higher or lower temperatures can be employedbut the above temperature range has been found to be most suitable.Higher temperatures result in sizeable product loss and vaporizerfouling under any practical conditions. Lower temperatures are not highenough to be coupled with a boron promoted hydrocarbon oxidation.

Vaporizer pressure is desirably maintained slightly greater than that inthe oxidation zones so as to'facilitate flow of vaporized cyclohexane tothe oxidation zones in orderto provide heat thereto. Illustrativevaporizer pressures are to 500 p.s.i.a.

The liquid passing from the vaporization zone'can be readily cooled,e.g., to about C. or less in orderto prevent further loss of peroxyoxygen contained therein. Alternatively, the liquid from thevaporization zone can be subjected to a controlled heat soaking inaccordance with the procedures of copending application Ser. No.593,646, filed Nov. 29, 1966 in order to accomplish a highly selectiveperoxy compound decomposition; that is, the preferred alternate.

It has been found through practice of the present invention thatsignificant and important improvements in process operation areachieved. To begin with, the proper regulation of the vaporization asabove indicated results in an overall increase in oxidation selectivelyof several percent. Still further, it has been found that operationalproblems which accompany conventional vaporization operation are to avery real degree minimized or completelyavoided.

Conventional vaporizer operation in such systems as here underconsideration usually involve residence times in excess of about 5minutes. In such conventional systems, high decomposition of peroxyoxygen occurs in the vaporizer and also there occurs the formation ofsolid or resinous deposition products which deposit on indirect heatexchange surfaces in the vaporizer thus causing very severe operationaldifficulties. Accompanying effects are lowered yields of the desiredproducts and also decrease in the quality of the ultimate product.

Through practice of the present invention the problems associated withconventional practices are minimized or completely overcome. l avoidingexcessive loss of peroxy oxygen in the vaporizer product yields areimproved and the formation of resinous deposits with accompanyingoperational difficulties is avoided to a very considerable degree.

The following examples illustrate the invention;

EXAMPLE 1 Cyclohexane is continuously oxidized in a series of fouragitated reactors. The oxidizing gas is air which is diluted by nitrogento an oxygen content by volume of about 8 percent. The oxidation iscarried out in each zone at a temperature of 165 C. and at a pressure ofabout p.s.i.a. The conversion in each of the four reactors is about 2.0percent of the cyclohexane equivalent (unreacted cyclohexane plusconverted cyclohexane leaving the vaporizer. Vapors comprisingcyclohexane, nitrogen, water, and the like are continuously removed fromeach reaction zone during the oxidation and these vapors are cooled andcondensed. Upon decantation the unreacted cyclohexane is recovered andrecycled to the oxidation.

The oxidation is carried out in the presence of meta boric acid which isincorporated with the cyclohexane fedto the first of the series ofreactors. Meta boric acid is employed in amount of 3 percent by weightof the cyclohexane fed to the first reactors.

Under the conditions of the oxidation heat is required in order tomaintain the oxidation temperature. This is provided by vaporizing aportion of the cyclohexane in the effluent from the last reactor andreturning this vapor to each of the four oxidation zones as needed inorder to maintain the oxidation temperature therein.

Specifically, effluent from the last oxidation zone containing by weight97.2 percent unreacted cyclohexane and 2.8 percent oxidation products ofwhich 0.75 percent by weight represents compounds containing peroxyoxygen is passed at 165 C. to a vaporization zone wherein the effluentis heated by indirect heat exchange to a temperature of 175 C. Pressurein this vaporization zone is 147 p.s.i.a. As a result of this heatingabout 60 percent of the unreacted cyclohexane in the effluent isvaporized and the resulting vapors are distributed among the fouroxidation zones in order to maintain the oxidizing conditions therein.Residence time in the vaporization zone is maintained at 1.5 minutes andthere occurs a decomposition of about 23 percent of the peroxy oxygencontained in the effluent. The liquid from the vaporization zone passesto a heat soaking zone wherein liquid is maintained at a temperature ofabout 165 C. for 20 minutes in order to complete the peroxy compounddecomposition under the conditions of optimum selectivity.

The reaction mixture is worked up in accordance with know procedures andan ultimate yield of cyclohexanol and cyclohexanone of 91 percent basedon cyclohexane reacted is achieved. The oxidation is continued forprolonged periods of the order of several months without operationaldifficulties due to a formation of deposits on heat exchange surfaces inthe vaporizer.

By way of contrast, wherein the vaporizer is similarly operated exceptthat the residence time therein is 8 minutes, about 60 percent of theperoxy oxygen compounds are decomposed and the total yield ofcyclohexanone and cyclohexanol based on cyclohexane converted drops to87 percent. Also, importantly there is a severe problem in operation ofthe vaporizer due to the accumulation of resinous deposits on the heatexchange surfaces therein. This accumulation becomes apparent after onlya few weeks of operation.

EXAMPLE 2 Example 1 is repeated except that the residence time in thevaporizer is maintained at 1 minute. Peroxide decomposition in thevaporizer drops to about 17.5 percent and the total yield ofcyclohexanol and cyclohexanone based on cyclohexane converted rises toabout 92.5 percent.

By way of comparison, the residence time in the vaporizer is increasedto about l0 minutes and about 64 percent of there is decomposed in thevaporizer. Exceedingly severe problems of deposition in the vaporizerare encountered and the overall process selectivity drops to about 85.2percent cyclohexanol plus cyclohexanone based on cyclohexane converted.

Reference is made to the drawing which accompanies the excessiespecification. From an analysis of this drawing the marked effect ofresidence time in the vaporizer upon peroxide compound decomposition canbe seen. In accordance with the present invention, this decomposition ismaintained less than 50 percent and preferably less than 30 percent. Atthe appropriate vaporizer temperatures of 170 to about 185 C. it can beseen that the residence time must be maintained less than-about 5minutes at the lower temperatures and at the higher temperatures theresidence time must be even lower in order to avoid excessivedecomposition of peroxide in the vaporizer.

What is claimed is:

1. In a process for the vaporization of unreacted cyclohexane from aborate ester containing cyclohexane oxidation reaction mixture whichalso contains 5 to 30 percent peroxy oxygen compounds, based on thetotal of cyclohexane oxidation products, the improvement which comprisesheating the mixture under a pressure of to 500 p.s.i.a. to a temperaturein the range of to C. for 0.5 to 5 minutes so that less than 50 percentloss of peroxy oxygen occurs and separating cyclohexane vapors duringsaid heating.

2. A method of claim 1 wherein the liquid after removal of cyclohexanevapor is heat soaked at a temperature of 150-l7 0 C.

3. A method of claim 1 wherein the separated cyclohexane vapor isrecycled to a cyclohexane oxidation step.

2. A method of claim 1 wherein the liquid after removal of cyclohexanevapor is heat soaked at a temperature of 150170* C.
 3. A method of claim1 wherein the separated cyclohexane vapor is recycled to a cyclohexaneoxidation step.